LV. The optical and electrical properties of single crystals of sodium niobate

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Abstract
The optical strains and dielectric constants of small single crystals of sodium niobate have been measured over the temperature range -190°c to +650°c. At room temperature the properties are consistent with an antiferroelectric structure, and for high fields applied perpendicular to the orthorhombic c axis, double hysteresis curves have been observed. At low temperature there is evidence of ferroelectric properties for fields applied parallel to the c axis. In the most perfect crystals the Curie temperature is below -209°c, but the ferroelectric phase may be induced by a high electric field. There is a long thermal hysteresis on this change and once transformed the crystals remain ferroelectric to -55°c. Above 350°c the symmetry is only pseudo-tetragonal, truly orthorhombic, and two minor structure changes, at 470°c and 518°c are observed, before the structure becomes cubic at 638°c.